A reflector antenna for a satellite or an array antenna which arrays a plurality of single elements has a larger size than a single antenna about dozens times. Accordingly, a minimum gap distance Rfar between a source antenna and a measuring antenna should satisfy Rfar=2 L2/λ in order to acquire a far distance radiation pattern. Here, L is a length of an aperture plane of the measuring antenna, and λ is a wavelength of an operating frequency. For example, a minimum gap distance of a far distance radiation pattern of an antenna, which is operated at 10 GHz frequency and has a length of 20 λ, is 24 m. Therefore, it is impossible to measure a radiation pattern in a smaller anechoic chamber than the minimum gap distance.
A field measuring method is used as an alternative method for the above method. However, the field measuring method is sensitive to weather, and may generate an interference and problem between conventional communication services. Moreover, in a case of an antenna for a military or a security, the field measuring method has a demerit that it is difficult to maintain a secret by being exposed to a spy satellite. There is a compact range method as another alternative method which generates a plane wave by transmitting a wave from the source antenna to a reflector at a measuring point. The compact range method needs a reflector antenna which is expensive and quite difficult to be attached or detached. The compact range method has problems that it is difficult to install and maintain the reflector since an area of the reflector increases at a low frequency band of hundreds of MHz, and it is difficult to satisfy a process error of μm at a high frequency above 100 GHz when the reflector is manufactured.
Meanwhile, a short field measuring method, which measures near electric field and converts the measured near electric field into a short distance pattern, may be considered. After measuring vertical and horizontal polarized waves at a distance of 3-10×, this method acquires a far distance pattern by performing a Fourier transform. However, this method needs a lot of resources since instrument member moving a probe needs precision.
Accordingly, it is required a method which measures a large antenna without installing a high cost system or an additional anechoic chamber where there is a conventional far distance region measuring anechoic chamber. A method for converting measured radiation pattern in a Fresnel region into a far distance radiation pattern is published after measuring a radiation pattern in a Fresnel region (a partial distance of R=2 L2/λ) which is a middle region between a far distance region and a short distance region. Here, after changing an angle of the measuring antenna upward or downward and measuring the radiation pattern at various cross sections, the measured data is summed based on an equation. It is proved that this method has a high accuracy through various published documents. However, this method has some demerits under a real measuring environment. Firstly, when an angle of a large antenna of dozens kilograms (kg) is changed upward or downward, a safety accident may occur due to the weight of the antenna. Further, if an angle of the measuring antenna is changed upward or downward, this method may generate a measuring error by receiving all unpreferable reflective waves which are reflected from an absorbing body attached to a bottom plane and a ceiling plane. Accordingly, generally, a measuring method for minimizing a reflective wave toward a bottom plane and a ceiling plane by using a source antenna having a high directivity is requested in a Fresnel region.